Evolution 14.3

Objectives Summarize Darwin’s theory of natural selection Compare and contrast artificial selection with natural selection Relate pesticide resistance in insects to natural selection

Population- a group of individuals of the same species living in the same area at the same time. Ex) an animal species from a mainland colonizes a chain of distant and isolated islands. The individuals of that species on each island make up a separate population. Populations on the different islands would adapt to their local environments. Over time, the isolated populations would become more and more different. Over many generations, the populations could become different enough to be separate species..

The evolution of finches on the Galápagos Islands is an example. These finches are known as "Darwin's finches" for their contribution to the theory of natural selection

There are 13 species of finches unique to the Galápagos islands. However, they most closely resemble one finch species living on the South American mainland. A reasonable hypothesis is that the islands were colonized by a single finch species that strayed from the mainland. This single species adapted to the varied habitats on the islands and eventually diversified into the 13 species seen today. A key characteristic of the finches is their beaks, which are adapted to specific foods available on the different islands

Darwin's theory proposes that these differences arose through natural selection. Darwin based his theory of natural selection on two key sets of observations. First, drawing from Malthus's ideas about humans, Darwin recognized that all species tend to produce excessive numbers of offspring. But in nature, resources are limited. The production of more individuals than the environment can support leads to a struggle for existence among the individuals of a population. In most cases, only a small percentage of offspring will survive in each generation. The rest are starved, eaten, frozen, diseased, unmated, or unable to reproduce for other reasons.

Darwin's second set of observations was his awareness of variation among the individuals of a population. Variation refers to differences among members of the same species.Variation Much of this variation is heritable and passes from generation to generation. This explains why siblings usually share more traits with one another and with their parents than they do with unrelated members of the same population.

Natural Selection Individuals with inherited traits that are best suited to the local environment are more likely to survive and reproduce than less fit individuals. When this process repeats over many generations, each new generation has a higher proportion of individuals with the advantageous traits.

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Darwin also reasoned that natural selection could eventually cause two isolated populations of the same species to become separate species as they adapted to their different environments. This would explain patterns such as those observed in the Galápagos finches.

Artificial Selection Artificial selection is the selective breeding of domesticated plants and animals to produce offspring with genetic traits that humans value.Artificial selection Darwin observed that breeders selected individuals with the desired traits as breeding stock. Breeders play the role of the environment, allowing only those plants or animals with desired traits to reproduce.

Pesticides—Natural Selection in Action Natural selection and the evolution it causes can be observable events. An example is the evolution of pesticide resistance in hundreds of insect species. Pesticides are poisons used to kill insects that are pests in crops and in homes

Darwin observed that artificial selection could produce a great deal of change in a species in a short time. He reasoned that over thousands of generations, natural selection could also cause major change

A relatively small amount of poison dusted onto a crop may kill 99 percent of the insects. But later sprayings are less and less effective. Figure traces how pesticide resistance evolves.

Figure By spraying crops with poisons to kill insect pests, humans have favored the reproduction of insects with inherited resistance to the poisons over those with no resistance.

Most survivors of the first pesticide treatments were insects with genes that somehow enabled them to resist the chemical attack. Their offspring inherited the genes for pesticide resistance. In each generation, the percentage of pesticide- resistant individuals in the beetle population increased. The population underwent evolutionary change that resulted in adaptation to a change in the chemical environment—the presence of the pesticide.

Two key points about natural selection First, natural selection is a process of "screening" traits that are available. A pesticide does not create resistant individuals, but selects for resistant insects that are already present in the population. Second, natural selection favors those characteristics in a varying population that fit the specific current, local environment. Pesticide resistance offered no advantage until pesticide application changed the local environment